diff --git a/library/include/modules/Units.h b/library/include/modules/Units.h
index 2718f357f..5578dc521 100644
--- a/library/include/modules/Units.h
+++ b/library/include/modules/Units.h
@@ -226,12 +226,12 @@ DFHACK_EXPORT extern const std::vector<int32_t> stress_cutoffs;
 DFHACK_EXPORT int getStressCategory(df::unit *unit);
 DFHACK_EXPORT int getStressCategoryRaw(int32_t stress_level);
 
-DFHACK_EXPORT void subtractActionTimer(df::unit *unit, int32_t amount, df::unit_action_type affectedActionType);
-DFHACK_EXPORT void subtractCategoryActionTimers(df::unit *unit, int32_t amount, df::unit_action_type_group affectedActionTypes);
-DFHACK_EXPORT void multiplyActionTimer(df::unit *unit, float amount, df::unit_action_type affectedActionType);
-DFHACK_EXPORT void multiplyCategoryActionTimers(df::unit *unit, float amount, df::unit_action_type_group affectedActionTypes);
-DFHACK_EXPORT void setActionTimer(df::unit *unit, int32_t amount, df::unit_action_type affectedActionType);
-DFHACK_EXPORT void setCategoryActionTimers(df::unit *unit, int32_t amount, df::unit_action_type_group affectedActionTypes);
+DFHACK_EXPORT void subtractActionTimer(color_ostream &out, df::unit *unit, int32_t amount, df::unit_action_type affectedActionType);
+DFHACK_EXPORT void subtractCategoryActionTimers(color_ostream &out, df::unit *unit, int32_t amount, df::unit_action_type_group affectedActionTypes);
+DFHACK_EXPORT void multiplyActionTimer(color_ostream &out, df::unit *unit, float amount, df::unit_action_type affectedActionType);
+DFHACK_EXPORT void multiplyCategoryActionTimers(color_ostream &out, df::unit *unit, float amount, df::unit_action_type_group affectedActionTypes);
+DFHACK_EXPORT void setActionTimer(color_ostream &out, df::unit *unit, int32_t amount, df::unit_action_type affectedActionType);
+DFHACK_EXPORT void setCategoryActionTimers(color_ostream &out, df::unit *unit, int32_t amount, df::unit_action_type_group affectedActionTypes);
 
 }
 }
diff --git a/library/modules/Units.cpp b/library/modules/Units.cpp
index 29afd00e9..eee359254 100644
--- a/library/modules/Units.cpp
+++ b/library/modules/Units.cpp
@@ -2041,7 +2041,7 @@ void setActionTimerCore(df::unit_action *action, int32_t amount) {
     }
 }
 
-void Units::subtractActionTimer(df::unit *unit, int32_t amount, df::unit_action_type affectedActionType)
+void Units::subtractActionTimer(color_ostream &out, df::unit *unit, int32_t amount, df::unit_action_type affectedActionType)
 {
     CHECK_NULL_POINTER(unit);
     for (auto action : unit->actions) {
@@ -2050,7 +2050,7 @@ void Units::subtractActionTimer(df::unit *unit, int32_t amount, df::unit_action_
     }
 }
 
-void Units::subtractCategoryActionTimers(df::unit *unit, int32_t amount, df::unit_action_type_group affectedActionTypes)
+void Units::subtractCategoryActionTimers(color_ostream &out, df::unit *unit, int32_t amount, df::unit_action_type_group affectedActionTypes)
 {
     CHECK_NULL_POINTER(unit);
     for (auto action : unit->actions) {
@@ -2064,18 +2064,30 @@ void Units::subtractCategoryActionTimers(df::unit *unit, int32_t amount, df::uni
     }
 }
 
-void Units::multiplyActionTimer(df::unit *unit, float amount, df::unit_action_type affectedActionType)
+bool validateMultiplyActionTimerAmount(color_ostream &out, float amount) {
+    if (amount < 0) {
+        out.printerr("Can't multiply action timer(s) by negative amount.\n");
+        return false;
+    }
+    return true;
+}
+
+void Units::multiplyActionTimer(color_ostream &out, df::unit *unit, float amount, df::unit_action_type affectedActionType)
 {
     CHECK_NULL_POINTER(unit);
+    if (!validateMultiplyActionTimerAmount(out, amount))
+        return;
     for (auto action : unit->actions) {
         if (affectedActionType != action->type) continue;
         multiplyActionTimerCore(action, amount);
     }
 }
 
-void Units::multiplyCategoryActionTimers(df::unit *unit, float amount, df::unit_action_type_group affectedActionTypes)
+void Units::multiplyCategoryActionTimers(color_ostream &out, df::unit *unit, float amount, df::unit_action_type_group affectedActionTypes)
 {
     CHECK_NULL_POINTER(unit);
+    if (!validateMultiplyActionTimerAmount(out, amount))
+        return;
     for (auto action : unit->actions) {
         auto list = ENUM_ATTR(unit_action_type, group, action->type);
         for (size_t i = 0; i < list.size; i++) {
@@ -2087,18 +2099,30 @@ void Units::multiplyCategoryActionTimers(df::unit *unit, float amount, df::unit_
     }
 }
 
-void Units::setActionTimer(df::unit *unit, int32_t amount, df::unit_action_type affectedActionType)
+bool validateSetActionTimerAmount(color_ostream &out, int32_t amount) {
+    if (amount <= 0) {
+        out.printerr("Can't set action timer(s) to non-positive amount.\n");
+        return false;
+    }
+    return true;
+}
+
+void Units::setActionTimer(color_ostream &out, df::unit *unit, int32_t amount, df::unit_action_type affectedActionType)
 {
     CHECK_NULL_POINTER(unit);
+    if (!validateSetActionTimerAmount(out, amount))
+        return;
     for (auto action : unit->actions) {
         if (affectedActionType != action->type) continue;
         setActionTimerCore(action, amount);
     }
 }
 
-void Units::setCategoryActionTimers(df::unit *unit, int32_t amount, df::unit_action_type_group affectedActionTypes)
+void Units::setCategoryActionTimers(color_ostream &out, df::unit *unit, int32_t amount, df::unit_action_type_group affectedActionTypes)
 {
     CHECK_NULL_POINTER(unit);
+    if (!validateSetActionTimerAmount(out, amount))
+        return;
     for (auto action : unit->actions) {
         auto list = ENUM_ATTR(unit_action_type, group, action->type);
         for (size_t i = 0; i < list.size; i++) {
diff --git a/plugins/fastdwarf.cpp b/plugins/fastdwarf.cpp
index 2a8f535ea..ea18437d6 100644
--- a/plugins/fastdwarf.cpp
+++ b/plugins/fastdwarf.cpp
@@ -83,7 +83,7 @@ DFhackCExport command_result plugin_onupdate ( color_ostream &out )
 
         if (enable_fastdwarf)
         {
-            Units::setCategoryActionTimers(unit, 1, df::unit_action_type_group::All);
+            Units::setCategoryActionTimers(out, unit, 1, df::unit_action_type_group::All);
         }
     }
     return CR_OK;